The invention relates to a thin-film strain gauge system there having an elastically deformable flexible substrate on which are provided an electrically insulating layer and thereon a structured resistance layer as well as an electrically readily conductive layer having a structure for the electrical contacting, as well as to a method of manufacturing such a strain gauge system.
For the measurement of the physical quantities mass, power, torque, acceleration, flow, pressure and pressure differential, measuring transducers with electric output signals are preferably used. Measuring transducers based on strain gauges comprise an elastic element (substrate) which is deformed by the measuring quantity, as well as a resistor by means of which this deformation is converted into an electrical signal. Such an electrical signal may then be amplified and be transmitted over large distances. It may be introduced into control loops, processed by computers or stored and it can also be shown easily on displays. The resistor (resistance layer) can achieve the conversion of the deformation into an electrical signal through a change of its resistance value.
As a material for the resistance layers there used metal alloys and semiconductors. For the measurement of the low measuring resistance variation, for example, four resistance paths formed from the resistance layer, hereinafter referred to as strain gauges, are combined to form a symmetrical wheatstone bridge.
The deviation from the bridge equilibrium is proportional to the elastic deformation of the strain gauge.
Thin-layer strain gauge systems are known in various forms in which in particular the properties of the electrically insulating layer between the elastic element and the resistance layer are of importance. For the electrically insulating layer various materials have been used which in practice, however, have proved to exhibit certain disadvantages.
For example, a thin-layer strain gauge system in which inorganic layers of oxides (Al.sub.2 O.sub.3, MgO or forsterite 2MgO.SiO.sub.2) are manufactured by means of RF cathode sputtering, electron beam evaporation or with heatable evaporators is known from DE-OS No. 27 41 055.
Thin-layer strain gauge systems in which inorganic layers of silicon oxide or silicon nitride are provided by means of plasma chemical vapour deposition are known from DE-OS No. 30 41 756.
European patent application No. 53 337 discloses thin-layer strain gauge systems in which the electrically insulating layer consists of polyimides, polyamide-imides or epoxy-modified polyimides, in which the layer materials are provided on the substrate as a solution, are centrifuged and are cured by a tempering treatment.
Various disadvantages are associated with the known electrically insulating layers.
Vapour-deposited layers or layers provided by cathode sputtering result in the coating of only a poor lateral quality. On substrates having microscopically small unevennesses this leads to short-circuits between the elastic substrate and the resistance layer.
The inorganic materials are moreover comparatively brittle and show haircracks even under small loads which adversely influence the long-life stability of the strain gauge. Under higher loads it results in a fracture which leads to interruptions of the resistance paths.
Although the above-mentioned organic layers may show a high maximum expansibility, they show poor creeping properties, in particular at higher temperatures.
BRIEF SUMMARY OF THE INVENTION
It is the object of the invention to improve the thin-film strain gauge system mentioned in the opening paragraph in such manner that it does not exhibit the above-mentioned disadvantages, hence to provide a thin-film strain gauge system whose electrically insulating layers lead to a good lateral coating, show a high maximum expansibility and at the same time are stable to above 300.degree. C. and which can compensate for the creeping of the resilient material (substrate).
According to the invention this object is achieved by having the electrically insulating layer consist of a plasma-polymerized material.
According to advantageous further embodiments of the invention the electrically insulating layer consists plasma-polymerized silicones or rigid-analogous silicones plasma-polymerized silicones or silizanes.
A method of manufacturing the thin-film strain gauge system according to the invention is characterized in that an electrically insulating layer of a plasma-polymerized material is formed by deposition from the gaseous phase on an elastically deformable flexible substrate, after which a resistance layer is provided on the polymer layer, is then structured to form at least a resistance track and electrical, thin-layer connections are formed on the structured resistance layer.
According to advantageous further embodiments of the method according to the invention the electrically insulating layer is formed in a plasma chemical vapour deposition device (PCVD device) in which at least one monomeric gas is introduced from which by high frequency excitation of the gas phase molecules polymerized Si:N:O:C:H-containing compounds can be formed which are deposited on the substrate present in the PCVD device. Advantageously hexamethyldisilazane is used as a monomeric gas.
The advantages which can be achieved by means of the invention consist in particular in that the electrically insulating layers of plasma-polymerized material have a good lateral coating. They show a good expansibility, are stable up to above 300.degree. C., are water repellant and chemically resistant. They readily adhere to all materials from which the elastic substrate is usually manufactured; moreover, there is also a good bonding to the overlying resistance layer. The layers can be prepared in any thickness suitable for the end in view between 0.2 .mu.m and 20 .mu.m. Moreover the possibility is obtained to of compensating for the creeping of the resilient material (substrate material) by suitable adjustment of the thickness and the composition of the layer.
The insulating layers according to the invention show excellent properties for the intended purposes: the likelihood of a short circuit to the elastic substrate (resilient member) is very small, which results in the advantage of a high yield in the manufacture of the strain gauge systems according to the invention. The electrically insulating layer according to the invention also shows a very good expansibility. Typical expansions occurring in pressure and force transducers are 1.times.10.sup.-3 m/m. In zones of non-uniform expansion, peaks in the expansion of 2.times.10.sup.-3 m/m can easily occur. If an overload strength should be ensured, expansions up to 4.times.10.sup.-3 m/m should be withstood without damage. The present insulation layers satisfy said requirements. The present electrically insulating layers are stable up to 300.degree. C. This is a particularly important advantage, for, on the one hand, high process temperatures occur during the manufacture of the expansion-sensitive resistance layer on the electrically insulating layer, and, on the other hand, the adjustment of the desired properties of the resistance layers usually requires a thermal after-treatment at which temperatures up to 300.degree. C. are typical. The present electrically insulating layer is also insensitive with respect to a high relative air humidity. In conventional strain gauge systems in which a resistance foil is provided on a synthetic resin support which is adhered to an elastic deformation carrier, the adhesive and the carrier material (substrate) consist of organic materials which expand at a high relative air humidity and so interfere with the measured signal of the measuring instrument.
The present electrically insulating layers have excellent bonding properties both to the elastic substrate and to the resistance layer.
In oxidic insulation layers, for example, the problem exists of the poor bonding and this difficulty must be overcome by providing additional layers which serve as bonding intermediate layers, which, for industrial manufacture, means an additional process step and hence higher cost.
The present electrically insulating layers also show a particularly good chemical resistance with respect to agressive liquids and gases. This is of advantage in regard to the reliability and the long-life stability of the measuring instrument. In no case may be electrically insulating layer be attacked by the chemicals used in the necessary photolithographic structuring process.
A further advantage of the present electrically insulating layers is that they can be manufactured to be very thin, having thicknesses of approximately 0.2 .mu.m to 20 .mu.m, in which, however, they are very free from pin-holes, so that they are very dense.
Apart from the increase of the process cost which occurs in providing thick layers in thin-film technology, a good thermal coupling between the elastic substrate and the resistance layer is of importance. An immediate bonding is also desired for an optimum transmission of the expansion profile of the elastic substrate to the strain gauge formed from the resistance layer.
A further advantage is that the present electrically insulating layers can be directly adjusted in their creeping properties. The slow deformation of the elastic substrate (spring member) under constant load is referred to as creeping. In good resilient materials the expansion at the surface changes, within five minutes after a variation of the load, by values between 0.01% and 0.05% and in this manner causes an error in the pressure and force measurements, respectively. In bonded strain gauge systems it is possible to compensate for said creeping, by particularly careful processing during adhering, for in measuring instruments manufactured in this manner the adhesive also fatigues and so produces an opposite creeping.
According to current teaching, such a compensation of the creepage of the elastic substrate (resilient member) is not possible with thin-film strain gauge systems (compare W. Ort. Wagen und Dosieren, 1979, No. 3, p. 86). However, it has surprisingly been found that with the thin-film strain gauge systems manufactured according to the invention it is possible, in contrast with the current teaching, to compensate for the creeping of the material of the elastic substrate. Different materials for the elastic substrates show different values both in the final value after 5 minutes and in the time variation of the creeping. By directed adjustment of the thickness and the proparation parameters the opposite creeping behaviour of the electrically insulating layers can be accurately adjusted so that the creepage error of the strain gauge system remains below 0.01% on typical materials for the elastic substrate, for example, steel. For this purpose reference is made to the data of the example.